The invention relates to an electrical machine, in particular to a commutator machine of the generic type.
Mechanically commutating commutator machines are widely used in motor vehicles. For example, high-power direct-current motors are used to start internal combustion engines in vehicles. The current is in this case passed via one or more brush pairs to a commutator, and from there to the armature winding of the machine. In this case, the so-called carbon brushes are generally composed of a sintered material, which mainly contains copper and graphite components. These carbon brushes and the commutator are subject to wear during operation. In this case, starters are typically designed for short-term operation with 30 000 to 60 000 switching cycles.
In order to save fuel, starters have recently been increasingly required for higher loads and for longer running times, for example for start-stop operation of the internal combustion engine. In this case, it is important that the lubrication and cleaning on the running surface of the commutator are optimally matched, in order to ensure a long carbon brush life. Because of the long running times of the brushes, the worn-away brush material as well as the excess sliding film on the commutator must be removed again. In order to clean the surface of the commutator well, various substances are mixed into the carbon brushes, which ensure that the surface of the commutator has a coating which, as far as possible, remains clean, while at the same time also providing lubrication.
For this purpose, the carbon brushes have a specific proportion of cleaning agents added to them, in a known manner. This ensures a certain amount of constant cleaning of the surfaces, but associated with a certain amount of wear of the carbon brushes, which increases correspondingly as the proportion of cleaning agents rises. A homogeneous distribution of the cleaning agent in all the carbon brushes in the machine results in the cleaning agent acting uniformly on all the carbon brushes, with the same carbon-brush contact pressure.
In order to ensure that the carbon brushes have a correspondingly longer life when there are a greater number of switching cycles, the wear behavior of the carbon brushes must be optimized by matching the lubrication and cleaning of the commutator as well as possible. In this case, it is known for the carbon brushes to be provided with at least two layer sections which extend over the height and width of the carbon brushes as far as their running surface. In order to improve the current commutation, one layer section is in this case provided with a higher proportion of graphite, and another layer section, as a power layer for carrying current, is provided with a higher proportion of copper. Furthermore, it has already been proposed for the carbon brushes to have a cleaning agent section in at least one of the abovementioned layer sections, whose cleaning agent proportion is higher than in the other layer section of the carbon brushes. Since, in this case as well, all the carbon brushes in the direct-current motor are of the same design, this also results in the effect of the various additives accumulating. The various parameters, such as wear, lubrication, commutation and the like, can therefore be adjusted relatively coarsely.
The aim of the present solution is to adjust one or more of the above-mentioned parameters on the carbon brushes considerably more accurately.